For example, such sensors are composed of an interrogation unit (itself consisting of an emitting part and of a receiving part) and of a surface acoustic wave temperature sensor commonly known as an SAW (surface acoustic wave) sensor. The interrogation system as well as the SAW sensor are furnished with an antenna suited to the working frequency band (ISM band 433 MHz, 868 MHz, 2.45 GHz, etc.) thereby making it possible to perform wireless interrogation of the sensor.
The mode of interrogation is as follows: The emitter of the interrogation system dispatches an interrogation signal (low-frequency temporal pulse of a carrier in the ISM band) to the SAW sensor. The SAW device is of resonator type thereby making it possible to access structures of reduced size.
If the emission signal exhibits a resonant frequency that is sufficiently close to the natural frequency of the SAW resonator, the latter starts to resonate while passing through a load period. A steady state oscillation is then set up at the natural resonant frequency of the SAW device. This resonant frequency is proportional to the speed of the surface wave in the resonant cavity which itself depends on the temperature which the resonator is at.
The sensor re-emits a signal at its resonant frequency which carries the information related to the quantity to be measured, for example the temperature.
The receiver of the interrogation system detects outside of the emission time span all or part of the SAW signal (damped oscillation) and extracts therefrom the information sought, for example the temperature, via suitable signal processing.
Typically, the resonator is composed of an interdigitated comb transducer, consisting of an alternation of electrodes of widths which repeat with a certain periodicity called the metallization period deposited on a piezoelectric substrate which may advantageously be quartz. The electrodes, advantageously aluminum electrodes (made by a photolithography process), have a small thickness compared with the metallization period (typically, a few hundred nanometers to a few micrometers). For example for a sensor operating at 433 MHz, the thickness of metal (aluminum) used can be of the order of 1000 angstroms, the metallization period and the electrode width possibly being respectively of the order of 3.5 μm and 2.5 μm.
One of the ports of the transducer is for example linked to a RadioFrequency (RF) antenna and the other is grounded. The field lines thus created between two electrodes of different polarities give rise to surface acoustic waves in the electrode overlap zone.
The transducer is a bi-directional structure, that is to say the energy radiated towards the right and the energy radiated towards the left have the same intensity. By disposing electrodes on either side of the transducer, these electrodes playing the role of reflector, a resonator is made, each reflector partially reflecting the energy emitted by the transducer.
If the number of reflectors is raised, a resonant cavity is created, characterized by a certain resonant frequency. This frequency depends firstly on the propagation speed of the waves under the grating, said speed depending mainly on the physical state of the substrate, and therefore sensitive for example to temperature. In this case, this is the parameter which is measured by the interrogation system and it is on the basis of this measurement that a temperature can be calculated.
It is recalled that the variation of the resonant frequency of a quartz-based resonator is determined by the following formula:f(T)=fO[1+CTF1(T−T0)+CTF2(T−T0)2]  (1)with f0 the frequency at T0, T0 the reference temperature (25° C. by convention), CTF1 the first-order coefficient (ppm/° C.) and CTF2 the second-order coefficient (ppb/° C.2).
It is also possible to reformulate this law by bringing in a temperature of inversion of the law (1), termed the turn-over temperature:f(T)=fTt+f0CTF2(T−Tturn-over)2  (2)with fTt the frequency at the turn-over temperature and Tturn-over the turn-over temperature;These quantities are given by the following equations:Tturn-over=T0−CTF1/2CTF2 fTt=f0[1−CTF12/4CTF2]  (3)
The law for the variation of the resonant frequency as a function of temperature is therefore a parabola; the temperature at which the frequency is a maximum (vertex of the parabola) is called the turn-over temperature.
It may be particularly beneficial to use two SAW resonators (W. Buff et al., “Universal pressure and temperature SAW sensor for wireless applications” 1997 IEEE Ultra. Symp. Proc.), inclined with respect to one another as illustrated in FIG. 1. In this case, a first resonator R1 for which the direction of propagation of the surface waves is along a direction X corresponding to one of the crystallographic axes of the crystalline substrate, is coupled to a second resonator R2, inclined by a certain angle α (which may typically be of the order of)20° with respect to the X axis, and therefore using another direction of propagation.
By inclining the second resonator with respect to the first resonator, it is endowed with a different sensitivity versus temperature. FIG. 2 illustrates such a behavior by presenting a typical spectral occupancy of the SAW temperature sensor (frequency in MHz as a function of temperature in ° C.).
In the chosen example, the first and second resonators have respectively a turn-over temperature in the vicinity of 150° C. and of 40° C. The space between the 2 lower curves and the 2 upper curves corresponds to the fabrication spread of the order of 250 kHz for this example.
The fact of using a differential structure presents several advantages. The first is that the frequency difference of the resonators is almost linear as a function of temperature and the residual non-linearities are corrected by calibrating the sensor. The other advantage of the differential structure resides in the fact that it is possible to circumvent the major part of the aging effects.
Generally, the temperature sensor uses two resonators R1 and R2 possessing two different directions of propagation.
The frequencies of the two resonators R1 and R2 may be written in accordance with equation (1):f1=f10[1+c1(ψ1)θ+c2(ψ1)2]f2=f20[1+c1(ψ2)θ+c2ψ2)θ2]  (4)with θ=T−T0 the deviation at ambient temperature, c1 and c2 the coefficients CTF1 and CTF2 and ψ the propagation angle.
The difference between the frequencies of the resonators R1 and R2 gives:Δf=f10−f20+θ(f10c1(ψ1))−f20c1(ψ2))+θ2(f10c2(ψ1))−f20c2(ψ2))
It is possible to rewrite the second-order equation in θ in the form:Δf=Δ0+sθ+εθ2  (5)with:Δf=f1−f2 Δ0=f10−f20 s=f10c1(ψ1)−f20c1(ψ2)ε=f10c2(ψ1)−f20c2(ψ2)  (6)where:
s represents the first-order temperature sensitivity
ε the coefficient of the term of order two
Δf the difference of the frequencies at the temperature θ (difference read out on interrogation)
Δ0 the difference of the nominal frequencies at the ambient temperature T0.
The aim of the calibration procedure is to determine the three terms: Δ0, s and ε so as to be able to calculate a posteriori the temperature from a measurement of the frequency difference Δf. Indeed, solving the second-degree equation in θ gives:T=T0+[−s+(s2−4ε(Δ0−Δf))1/2]/2ε  (7)
In order to facilitate the step of extracting the temperature on the basis of a measurement of difference in resonant frequency, three calibration coefficients a0, a1, a2 which make it possible to calculate the frequency are defined with the aid of equation (8):T=a0+(a1+a2Δf)1/2  (8)
The calibration is an operation consisting in determining the coefficients a0, a1 and a2; this operation nevertheless costs a great deal of time since it makes it necessary to measure for each sensor the frequency difference between the two resonators at three different temperatures as a minimum and moreover requires a serialization of each sensor (corresponding to the identification for each sensor of a sensor-calibration coefficients pair).
It is possible for example to envisage storing the calibration coefficients a0, a1, a2 in the interrogation system. Should there be a change of sensor, this configuration requires storage of the new coefficients in the interrogation system.
Together, these constraints are prohibitive in certain cases where objectives of low costs must be achieved.
Generally, the error in the evaluation of the temperature dθ measured with an SAW sensor with two resonators is given to first order by the following equation:dθ=(dΔI+dΔ0)/Sθ+θSθ/Sθ  (9)where                dΔI is the precision of reading the frequency at the interrogation system level        dΔ0 is the spread of the frequency difference between the 2 resonators at the temperature T0         Sθ is the typical sensitivity of the sensor (kHz/° C.)        dSθ is the sensitivity spread from one sensor to another        θ is the temperature deviation with respect to T0.        
If the error dθ is estimated for a given temperature T0+θ on the basis of the typical values of the parameters dΔI, dΔ0, Sθ and dSθ, an unacceptable value (in relation to the requirements of the applications) is obtained, which makes the calibration operation inescapable.
The largest contribution (greater than 80%) is related to the spread in the frequency difference.
The elements obtained after cutting blanks from the substrate on which the surface wave devices are made and bearing either a single resonator or a set of resonators carrying out the sensor function are called chips. It has already been envisaged to decrease the spread in the frequency variation, by tailoring each of the resonators by quartz or aluminum etching or by silica etching (passivation layer). This method requires the development of a specific process since it is necessary to adjust the frequency of the two resonators on the same chip in an independent manner. The etching precision must moreover be very high in order to meet the frequency spread requirements (typically: +/−5 kHz i.e. control of the order of +/−1 ångström, which is a difficult objective to achieve).